Refrigerating system of temp humidity chamber contains 2 single refrigerating circles and 1 double refrigeration cycle and Three components cold cycle ring.
A cascade refrigeration cycle system is used for obtaining low temperatures below -20 degrees C. And obtaining low temperature and using two stage compression cycle to obtain lower temperature. Because:
1. The limitation of the thermal physical properties of refrigerants
The temperature of refrigerant now constant temperature and humidity test machine in the single-stage refrigeration cycle basically is used in R404A, in an atmosphere of the evaporation temperature is -46.5 DEG C (R22/-40.7 C), but the air cooling condenser heat transfer temperature about 10 degrees Celsius (usually in the forced air cooling cycle, the evaporator and the temperature inside the box), is only for the -36.5 low temperature, of course, through the evaporation pressure to reduce the compressor, can reduce the minimum evaporation temperature of refrigerant R404A to -50 DEG C; so to get 50 degrees and under low temperature should be used in refrigeration cycle and low temperature refrigerant refrigerant cascade, producing 50 C ~ 80 low temperature, low temperature refrigerant is generally used in a R23 under the pressure of the evaporation temperature is -81.7 DEG C.
2.Restriction of cyclic pressure ratio of single stage compression steam refrigeration
The lowest temperature evaporation stage vapor compression refrigeration machine, condensing pressure mainly depends on its compression ratio. And the condensing pressure refrigerant refrigerant by category and environmental media (such as air or water) temperature, under normal circumstances, it is in the range of 0.7~1.8Mpa. The compression ratio and the condensing pressure and evaporation pressure. When the condensate pressure is constant, with the decrease of evaporation temperature, evaporation pressure also decreased, so that the compression ratio increases, it will increase the exhaust temperature of the compressor, lubricating oil thinning, the lubrication condition is bad, serious or even carbon and scuffing phenomenon; on the other hand, the increase of the compression ratio will lead to gas compressor transmission coefficient is reduced, the refrigeration amount is reduced, the actual compression process from isentropic process farther, compressor power increases, the refrigeration coefficient decreased, the economy decreased. Some will have the following effect.
A.The lower the evaporation temperature is, the lower the evaporative pressure of any refrigerant is. Too low evaporation pressure may sometimes cause the compressor to inhale or make the outside air enter the refrigeration system.
B.When the evaporation temperature is too low, some commonly used refrigerants have reached the solidification temperature, and the refrigerant flow can not be realized.
C.The evaporation pressure decreases, the specific volume of refrigerant increases, the mass flow rate of refrigerant decreases, and the cooling capacity decreases greatly. In order to obtain the required refrigerating volume, the suction volume must be increased to make the volume of the compressor too large.
3.Limitation of heat dissipation of compressor coil
Single stage compressor, at -35 degrees Celsius, because the compressor coil is empty in the middle of rotary compressor, this creates a problem, -35 DEG C, the compressor is low value is negative, which is also produced a vacuum, so the coil top heat had no way dispersed, so compressor surface is very cool, but in fact, his temperature is very high. (because the vacuum heat insulation medium is the best)
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